With the explosion of the information processing industry more and better methods for storing data have been required. One of the most effective methods currently in use utilizes magnetic media deposited on flexible disks, known as "floppy" disks. Floppy disks are enclosed for their protection and alignment in rectangular, usually square, envelopes known as "jackets". The present invention is primarily concerned with the methods and apparatus for manufacture of the floppy disk jackets.
Floppy disk jackets are ordinarily constructed utilizing of a polymeric material known as polyvinyl chloride or "PVC". This material is prepared in thin sheets which are pre-cut to form the outline of a unfolded jacket. The precut sheets are then folded into the appropriate shape (a square envelope) to enclose the floppy disk. The folding, which is ordinarily done at the center, both side portions and the front flap of the jacket, has been accomplished in the past either using a technique known as "cold folding" or by a technique known as "warm folding". Both methods are presently used to make floppy disk jackets in the industry.
The field of the present invention is in relation to the warm folding methods. The warm folding is the preferred method in the industry. The basis for warm folding stems from the fact that PVC and similar plastics and vinyls must be stress-relieved before folding. Stress relief before folding is essential to creating a long-life stable fold. The PVC must enter a quasi-molten state before mechanically stressing the material. PVC's may contain a high number of hard plastic components that may become very brittle under stress. If attempts are made to cold fold, or apply mechanical stress before the molten state obtains, the material may tear and pull microscopicly at the point of mechanical stress, creating microcracks on the edge. Depending on the material's chemical composition, PVC's require a critical amount of heat and time to absorb that heat before the material reaches the quasi-molten state.
Various methods have been utilized to deliver heat to the fold area to soften the material for successful folding. These have included hot air flow, high intensity infrared lamps and electrically resistive heater bars. The air flow method is inefficient in that the heat application is not precisely localized. The infrared lamp method suffers from reflectance differentials, due to material surface differences, and thus the heat delivery is not precisely predictable. Heat is delivered to areas other than the selected fold area. This may occasionally lead to buckling or other degradation of the material surrounding the fold. It has also been found that heater bars, using conductive rather than radiative energy transference, are more electrically efficient and may be more precisely controlled as to temperature and duration. Thus, the present invention is directed at heater bar methods and apparatus.
Prior art warm folding methods are described in U.S. Pat. No. 4,239,572 issued to K. Tomita on Dec. 16, 1980 and in Applicants' own prior patent, U.S. Pat. No. 4,487,218 issued May 8, 1984. Each of these references discloses a method of manufacturing floppy disk jackets or the like utilizing a warm folding technique. However, due to the limitations of the materials, the speeds and accuracies of the prior art methods are restricted. There has remained significant room for improvement in the areas of speed of manufacture and precision of folding.